This invention relates to the provision of electromagnetic interference and electrostatic discharge shielding of an electronic system and, more particularly, to a guide assembly for circuit cards which are part of such an electronic system.
Modern electronic systems equipment, such as for telecommunications purposes, is often constructed as modular circuit cards inserted into guide slots of mechanical card cages, or shelf units, for engagement with a backplane mounted to the shelf unit at the inward ends of the guide slots. The outward edge of each circuit card is fastened to a faceplate assembly which holds a latch for releasably retaining the circuit card in the shelf unit. Although the circuit cards are relatively closely packed, there are still gaps between adjacent faceplate assemblies. There are also gaps in the faceplate assemblies to allow portions of the latches to pass therethrough. All of these gaps allow electromagnetic interference radiation to leak therethrough. In addition, the gaps allow electrostatic discharge to pass therethrough, possibly causing damage to circuit components mounted on the circuit cards. A need therefore exists to provide shielding against electromagnetic interference and electrostatic discharge for such electronic systems equipment.
It is known to provide such shielding by encasing the electronic components within a conductive housing. While this"closed door" approach is satisfactory, there exists a need to provide such shielding under"open door" conditions, for example where a door to the enclosure is open to allow access to the equipment for maintenance purposes.
These needs have been addressed, for example, by a shielding assembly disclosed in co-pending U.S. patent application Ser. No. 09/309,930, filed May 11, 1999, and assigned to the assignee of this invention. As disclosed in the referenced application, each faceplate assembly secured to a circuit card includes an elongated conductive U-shape bracket member extending at least the length of the circuit card edge with the base wall of the U-shape being orthogonal to the circuit card. Secured to one side wall of each of the bracket members is a respective resilient compressible conductive gasket which extends the length of its respective bracket member. Each bracket member has its respective gasket secured to the same corresponding side wall as all the other bracket members so that between each adjacent pair of bracket members there is a single respective gasket. This seals all the gaps between adjacent faceplate assemblies.
While the aforedescribed assembly is effective, it has been found that the circuit card faceplate width can have dimensional variation that is considered acceptable in the manufacture process. However, this accumulation of variation (tolerance build-up) along with the springiness of the gaskets between adjacent faceplates can decrease the available opening for a circuit card to plug in between previously inserted cards. The tolerance build up is such that the first few circuit cards can be plugged in, but as there are more circuit cards in an adjacent group, the faceplates and gaskets start flaring out like an"accordion". The build up can be such that the last few circuit cards are difficult or impossible to install. Accordingly, a need exists to overcome this problem without at the same time diminishing the shielding effect of the aforedescribed assembly.